Photopolymerizable composition, photoinitiator mixture and curing process

ABSTRACT

The present invention relates to a photopolymerizable composition comprising a polymerizable, polyethylenically unsaturated compound, benzil and/or fluorenone, a specific biphenyl ketone such as para-phenyl benzophenone, and a reducing agent such as a tertiary amine. The present composition permits rapid and complete curing. Moreover, in the photopolymerization of compounds sensitive to air inhibition non-tacky surfaces are obtained. The effect observed is based on a synergism between the present photoinitiators. The advantages of the invention are most manifest in curing relatively thick layers of materials.

BACKGROUND OF THE INVENTION

Prior art photoinitiator systems comprising an aromatic carbonylcompound and a reducing agent are known from U.S. Pat. No. 3,759,807. Init are enumerated a great many aromatic carbonyl compounds, includingacetophenone, 4-tolyl acetophenone, benzophenone, 3-tolyl benzophenone,fluorenone, antraquinone, xanthone and thioxanthone. Reducing agentsmentioned includes amines, viz. tertiary amines such as triethanolamineand methyldiethanolamine. Photopolymerizable compositions are also knownfrom British Patent Specification No. 1,408,265, which also mentions agreat many aromatic carbonyl compounds, including benzil, benzilderivatives, naphthyl, phenanthraquinone, benzophenone, benzoin, benzoinethers and fluorenone.

Although these prior art photoinitiator systems may be satisfactorilyapplied for various purposes, a particular problem exists in completelyand rapidly curing material having a thickness of 1-20 mm. Moreover, inthe case of polymerizable, polyethylenically unsaturated compounds suchas methacrylate resins that are sensitive to air inhibition, there isthe problem of insufficient surface hardening, which manifests itself ina tacky surface. For optimum properties, there are two requirements: thematerial should be both thoroughly cured and of a good surface hardness.

Often the known photoinitiator systems satisfy only one of the tworequirements. In actual practice, it has therefore often been foundnecessary to isolate the surface to be cured from the ambient air bycovering it with a transparent film to obtain a satisfactory surfacehardness and/or to use prolonged exposure times for thorough curing.

SUMMARY OF THE INVENTION

The photopolymerizable composition according to the invention isnon-tacky, rapid setting, and hard curing, and comprises:

a. a polymerizable, polyethylenically unsaturated compound;

b. benzil and/or fluorenone;

c. A biphenylyl ketone of the formula: ##STR1## wherein: R₁ =hydrogen,an alkyl group having 1-8 carbon atoms, an alkoxy group having 1-10carbon atoms, an alkyl carbonyl group having 2-12 carbon atoms or ahalogen atom; and

R₂ =an alkyl group having 1-12 carbon atoms, phenyl, naphthyl, furyl orthiophenyl, which groups may be substituted or not; and

d. a reducing agent capable of reducing the aromatic carbonyl compoundswhen they are in the excited state.

It has been found that the use of the above-mentioned compositionpermits rapid and complete curing of relatively thick layers of thepolymerizable polyethylenically unsaturated compound. In thephotopolymerization of compounds sensitive to air inhibition it ispossible, moreover, to obtain non-tacky surfaces. The use of only benziland/or fluorenone generally leads to better curing but to a lowersurface hardness than the sole use of a corresponding amount of thepresent biphenylyl ketones.

As far as these aspects are concerned, it was found that a correspondingamount of a mixture of these aromatic carbonyl compounds results in bothrapid curing and high surface hardness of the polymerizable compound.Moreover, it has been surprisingly found that the result is not merelycumulative but is synergistic, and that this synergistic effect is notproduced by other combinations of the above-mentioned initiators.

The present composition contains a polymerizable, polyethylenicallyunsaturated compound. Examples thereof include polymethacrylates andpolyacrylates, the acryl being built in as reactive end group in apolymer chain, via hydroxy, amino, carboxy, isocyano or epoxy groups.Examples of this last mentioned type of compound includepolyacryl-modified polyesters, polyamides, polyethers, polyurethanes andpolyvinyl resins. Optionally, the polyethylenically unsaturated compoundmay be mixed with a co-polymerizable unsaturated monomer such as acrylicesters and methacrylic esters, acrylic and methacrylic amides andacrylic and methacrylic nitriles, for instance acrylonitrile, methylmethacrylate and 2-ethylhexyl acrylate. To that end, use may further bemade of vinyl esters and vinylidene esters, vinyl ethers and vinylketones, such as vinyl acetate, vinyl propionate, N-vinyl-pyrrolidoneand vinylbutyl ether.

Moreover, monomers may be added that have more than one unsaturatedterminal group. Examples thereof include diallyl phthalate, diallylfumarate, ethylene glycol dimethacrylate, pentaerythritol triacrylate,trimethylol propane triacrylate, neopentyl glycol dimethacrylate andpolyethylene glycol diacrylate. The monomers that may be used includesuitable copolymerizable hydrocarbons, such as styrene, vinyl tolueneand divinyl benzene.

The present composition also may contain a mixture of an unsaturatedpolyester resin and a monomeric compolymerizable compound, such asstyrene. The unsaturated polyester resins may be prepared in a knownmanner from unsaturated polycarboxylic acids or their anhydrides, suchas maleic anhydride or fumaric acid, and generally in the presence ofaromatic and/or saturated aliphatic dicarboxylic acids such as phthalicanhydride, tetrahydrophthalic anhydride, adipic acid and the like andpolyols, such as ethylene glycol, diethylene glycol, propylene glycoland the like, and after the polyester preparation the terminalcarboxylic groups may be further reacted to completion with reactiveepoxides, such as phenylglycidyl ether.

The present photoinitiator mixture contains a mixture of aromaticcarbonyl compounds which at least consists of:

a. benzil and/or fluorenone; and

b. a biphenyl ketone of the formula: ##STR2## wherein: R₁ =hydrogen, analkyl group having 1-8 carbon atoms, an alkoxy group having 1-10 carbonatoms, an alkyl carbonyl group having 2-12 carbon atoms or a halogenatom; and

R₂ =an alkyl group having 1-12 carbon atoms, phenyl, naphthyl, furyl orthiophenyl, which groups may be substituted or not.

The group R₂ may be substituted, for instance with the groups denoted byR₁.

Examples of suitable biphenylyl ketones include: para-phenylacetophenone, para-phenyl benzophenone, para-tolyl benzophenone, para,para'-diacetyl biphenyl, para-(para-chlorophenyl)-benzophenone,para-(para-methoxyphenyl)-benzophenone, α-furyl biphenylyl ketone,α-naphthyl biphenylyl ketone, para-tertiary butyl phenyl biphenylylketone, para-methoxy phenyl biphenylyl ketone and 2,4-dichlorophenylbiphenylyl ketone.

As biphenylyl ketone the composition preferably contains para-phenylbenzophenone. As regards benzil and fluorenone it should be noted thatboth have a triplet energy of 53-54 kcal/mole, whereas p-phenylbenzophenone has a triplet energy of about 60 kcal/mole.

The present composition generally contains 0.1-1.5% by weight andpreferably 0.5-1.0% by weight of the aromatic ketone mixture. The molarratio of the biphenylyl ketone to the benzil and/or fluorenone isgenerally in the range of from 0.25 to 9 and preferably in the range offrom 0.5 to 3.0.

The present photopholymerizable compositions should also contain areducing agent. It should be capable of reducing the aromatic ketoneswhen they are in the excited state and moreover, after oxidationthereof, of initiating the polymerization of the polyethylenicallyunsaturated compound via intermediately formed radicals.

Suitable reducing agents are described in, int.al., British Patentspecification No. 1,408,265. The compounds described have the formula##STR3## wherein M is an element of group Vb of the Periodic Table ofthe Elements and the groups R, which may be the same or different, arehydrogen atoms, hydrocarbon groups, substituted hydrocarbon groups orgroups in which two groups R together with the element M form a cyclicring system, no more than two of the groups R being hydrogen atoms.Where the element M is attached directly to an aromatic group R, atleast one of the other groups has an ##STR4## group attached to M.

It is preferred that as reducing agent there should be used an amine,more particularly a tertiary amine. Examples of suitable amines includetriethanolamine, triisopropanolamine, N-methyl diethanolamine,N,N-dimethyl ethanolamine, N,N-dimethyl isopropanolamine, N-hydroxyethylpiperidine, N-hydroxyethyl morpholine, bis(2-hydroxymethyl) oleylamine,N,N,N',N'-tetramethyl-1,3-diaminopropane, N,N-dimethyl benzylamine,N,N-dimethyl aniline, 4-dimethyl aminobenzoate, dimethyl aminoethylacrylate and dimethyl aminoethyl methacrylate.

The present composition generally contains 0.1-4.5% by weight,preferably 0.25-2.0% by weight of reducing agent. The weight ratiobetween the mixture of aromatic ketones and the reducing agent isgenerally in the range of 1:3 to 3:1, preferably 2:1 to 1:2.

The present compositions are cured by exposure to radiation in a knownmanner. To that end use is generally made of a radiation source emittingradiation having a wave length in the range of 200 to 500 nm. Suitableradiation sources include medium-pressure mercury lamps.

The invention also relates to the curing of relatively thick layers ofmaterial (i.e., having a thickness of 1-20 mm, more particularly 4-10mm) based on the present photopolymerizable compositions. When thephotopolymerizable compositions according to the invention are treatedin this way their advantages will be the most manifest. Cured reinforcedlaminates based on the present composition and having a thickness of4-10 mm possess excellent properties.

As reinforcing material there may be used glass fibers in the form ofbundles (rovings), glass mats, glass fibers, or glass webs, optionallyin combination with layers of other (textile) materials.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is further described in the following examples. Measuresof the surface hardness and the degree of curing of the laminates werethe non-tackiness of the boundary surfaces and the Barcol indentationhardness (type 934-1).

The maximum possible indentation hardness depends on type of resin,degree of curing, type of reinforcement and degree of reinforcement. Allamounts are expressed in percentage by weight.

EXAMPLES 1 THROUGH 6

A resin solution was prepared consisting of 97% of technical4,4'-isopropylidene bis(phenylene oxyethyl methacrylate), 2% oftriisopropanolamine and 1% of aromatic ketone(s). The solution obtainedwas used for preparing glass-reinforced laminates. They were built up ofthe resin and four layers of glass fiber mat. The thickness of thelaminates was about 4 mm and the glass content 54%. The material wascured with a Philips medium-pressure mercury lamp of the HTQ-4 typehaving an intensity of radiation of 20 W per cm. The distance betweenthe lamp and the laminate was 20 cm and the curing time four minutes.

The Examples 1 through 4 in Table 1 show the results obtained withbenzil, para-phenyl benzophenone and mixtures of these carbonylcompounds. The Examples 5 and 6 are for comparison, use being made of aresin solution composed of 98.5% of said bis-methacrylate, 1%triisopropanolamine and 0.5% of aromatic ketone.

                                      TABLE 1                                     __________________________________________________________________________                        state of laminate surface                                                                 indentation                                                       tackiness   hardness                                      Example                                                                            Percent Aromatic ketone(s)                                                                   upper under upper                                                                             under                                     __________________________________________________________________________    1    1.00% p-phenyl benzophenone                                                                  tack-free                                                                           tack-free                                                                           47.5                                                                              37.3*                                     2    0.66% p-phenyl benzophenone                                                                  tack-free                                                                           tack-free                                                                           51.1                                                                              52.4                                           0.33% benzil                                                             3    0.50% p-phenyl benzophenone                                                                  tack-free                                                                           tack-free                                                                           48.4                                                                              47.6                                           0.50% benzil                                                             4    1.00% benzil   tacky tack-free                                                                           46.7                                                                              48.9                                      5    0.50% benzil   tacky tacky-free                                                                          30.9                                                                              39.9*                                     6    0.50% p-phenyl benzophenone                                                                  tacky wet   33.8                                                                              not                                                                           measur-                                                                       able                                      __________________________________________________________________________     *insufficiently cured                                                    

The examples 2 and 3 show the synergistic effect which results from theuse of the present photoinitiator system.

EXAMPLES 7-12

For comparison the experiments of the Example 1 through 4 were repeatedexcept that use was made of different aromatic carbonyl compounds.

The results are summarized in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                        state of laminate surface                                                              indentation                                      Aromatic carbonyl compound                                                                        tackiness                                                                              hardness                                         Example                                                                            (%)            upper                                                                             under                                                                              upper                                                                             under                                        __________________________________________________________________________     7   1.00% benzophenone                                                                           tacky                                                                             wet  41.3                                                                              not                                                                           measur-                                                                       able*                                         8   1.00% benzoin  tacky                                                                             wet  45.7                                                                              not                                                                           measur-                                                                       able*                                         9   1.00% benzil dimethyl ketal                                                                  tacky                                                                             tack-free                                                                          47.8                                                                              34.6*                                        10   0.33% benzophenone                                                                           tacky                                                                             tacky                                                                              41.2                                                                              23.0*                                             0.66% p-phenyl benzophenone                                              11   0.33% benzoin  tacky                                                                             tack-free                                                                          50.2                                                                              40.3*                                             0.66% p-phenyl benzophenone                                              12   0.33% benzil dimethyl ketal                                                                  tacky                                                                             wet  40.3                                                                              not                                               0.66% p-phenyl benzophenone measur-                                                                       able*                                        __________________________________________________________________________     *insufficiently cured                                                    

The results in Table 2 demonstrate that the combination of para-phenylbenzophenone and other known photoinitiators such as benzophenone,benzoin, and benzil dimethyl ketal does not produce any synergisticeffect at all.

EXAMPLES 13 THROUGH 22

Laminates were prepared that were made up of the resin described in theExamples 1 through 6 and four layers of glass mat. The laminatethickness was 4 mm, the glass content 26%. Curing was effected in themanner indicated in the Examples 1 through 6 in the presence of 2% oftriisopropanolamine, as was the testing of the laminate properties.Table 3 shows the results.

                                      TABLE 3                                     __________________________________________________________________________                        state of laminate surface                                                               indentation                                     Aromatic carbonyl compound                                                                        tackiness hardness                                        Example                                                                            (%)            upper                                                                              under                                                                              upper                                                                             under                                       __________________________________________________________________________    13   0.50% benzil   tack-free                                                                          tack-free                                                                          35.1                                                                              36.4                                             0.50% p-phenyl benzophenone                                              14   0.50% fluorenone                                                                             tack-free                                                                          tack-free                                                                          40.5                                                                              39.0                                             0.50% p-phenyl benzophenone                                              15   0.50% benzil   tacky                                                                              tack-free                                                                          34.1                                                                              37.1                                             0.50% fluorenone                                                         16   0.50% benzil   tacky                                                                              tack-free                                                                          35.6                                                                              39.8                                             0.50% benzophenone                                                       17   0.50% benzil   tacky                                                                              tack-free                                                                          33.3                                                                              37.3                                             0.50% benzil dimethyl ketal                                              18   0.50% benzil   tacky                                                                              tack-free                                                                          42.0                                                                              39.5                                             0.50% benzoin butyl ether                                                19   0.50% benzil   tacky                                                                              tack-free                                                                          35.5                                                                              38.4                                             0.50% acetophenone                                                       20   0.50% benzil   tacky                                                                              tack-free                                                                          40.1                                                                              42.2                                             0.50% 1-phenyl propane                                                        1,2-dione-2-oxime-o-                                                          benzoate                                                                 21   0.50% benzil   tacky                                                                              tack-free                                                                          36.8                                                                              41.4                                             0.50% 2-hydroxy-2-benzoyl                                                     propane                                                                  22   0.50% benzil   tacky                                                                              tack-free                                                                          39.9                                                                              42.5                                             0.50% 2-hydroxy 2(p.iso-                                                      propyl benzoyl)                                                               propane                                                                  __________________________________________________________________________

Only the compositions of the Examples 13 and 14 are the ones accordingto the invention. The others serve for comparison. The data show thatsatisfactory results are only obtained with the photoinitiator systemaccording to the invention. Combinations of benzil with aromaticcarbonyl compounds other than the present biphenyl ketones lead toinsufficient surface hardening.

EXAMPLES 23 THROUGH 30

The procedure used in the Examples 13 through 22 was repeated, with theexception that as aromatic carbonyl compound there was used a mixture of0.5% of benzil and 0.5% of the biphenylyl ketones mentioned in Table 4and the distance between the lamp and the laminate was 15 cm. Theresults are given in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                       state of laminate surface                                                                indentation                                                        tackiness  hardness                                        Example                                                                            biphenylyl ketone                                                                           upper                                                                              under upper                                                                             under                                       __________________________________________________________________________    23   p-phenyl acetophenone                                                                       tack-free                                                                          tack-free                                                                           38.3                                                                              35.6                                        24   p,p'diacetyl biphenyl                                                                       tack-free                                                                          tack-free                                                                           38.0                                                                              38.8                                        25   α-furyl biphenylyl ketone                                                             tack-free                                                                          tack-free                                                                           35.6                                                                              37.4                                        26   α-thiophenyl biphenylyl                                                               tack-free                                                                          tack-free                                                                           40.3                                                                              35.1                                             ketone                                                                   27   α-naphthyl biphenylyl                                                                 tack-free                                                                          tack-free                                                                           43.0                                                                              42.8                                             ketone                                                                   28   p-tolyl benzophenone                                                                        tack-free                                                                          tack-free                                                                           38.3                                                                              35.6                                        29   p-methoxyphenyl                                                                             tack-free                                                                          tack-free                                                                           34.3                                                                              40.7                                             benzophenone                                                             30   p-chlorophenyl benzo-                                                                       tack-free                                                                          tack-free                                                                           37.3                                                                              36.1                                             phenone                                                                  __________________________________________________________________________

The above results show that the above-described synergism also occurs inthe case of biphenylyl ketones other than paraphenyl benzophenone.

EXAMPLES 31 THROUGH 39

Glass-reinforced laminates were constructed from twelve layers of glassfiber mat and from the resin described in Examples 1 through 6. Thelaminate that was formed had a thickness of about 10 mm and a glasscontent of 55%.

In the resin there was dissolved a mixture of aromatic ketones made upof 45% of benzil and 55% of p-phenyl benzophenone. As a reducing agentthere was added triisopropanolamine in an amount such that the weightratio of the amine to the ketone mixture was 2.

The laminates were exposed to radiation from a Philips medium-pressuremercury lamp of the HTQ-4 type. The distance between the lamp and thelaminate was 20 cm, and the radiation times were 8, 10 and 12 minutes,respectively. The results are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                        state of laminate surface                                                               indentation                                     Amount of     radiation                                                                           tackiness hardness                                        Example                                                                            ketone mixture                                                                         time (min.)                                                                         upper                                                                              under                                                                              upper                                                                             under                                       __________________________________________________________________________    31   1.00%    8     tack-free                                                                          tack-free                                                                          45.2                                                                              47.0                                        32   1.00%    10    tack-free                                                                          tack-free                                                                          46.1                                                                              44.1                                        33   1.00%    12    tack-free                                                                          tack-free                                                                          45.9                                                                              44.8                                        34   0.50%    8     tack-free                                                                          tack-free                                                                          42.7                                                                              46.7                                        35   0.50%    10    tack-free                                                                          tack-free                                                                          43.1                                                                              43.9                                        36   0.50%    12    tack-free                                                                          tack-free                                                                          45.6                                                                              45.6                                        37   0.20%    8     tack-free                                                                          tacky                                                                              38.4                                                                              0.0                                         38   0.20%    10    tack-free                                                                          tack-free                                                                          37.6                                                                              47.1                                        39   0.20%    12    tack-free                                                                          tack-free                                                                          43.1                                                                              49.7                                        __________________________________________________________________________

The above data show that satisfactory results can be obtained withinacceptable radiation times even with the use of (very) small amounts ofthe photoinitiator system.

EXAMPLES 40 THROUGH 43

Glass-fiber reinforced tubes having a diameter of 88 mm and a wallthickness of 4 mm were made by passing a bundle of glass fibers throughan impregnating bath containing 4,4'-isopropylidene-bis(phenyleneoxyethyl-methacrylate) in which there had been dissolved 1% of aromaticketone(s) and 2% of triisopropanolamine and winding said bundle onto arotary steel cylinder.

Subsequently, the impregnated product was exposed to 15 minutes'radiation from a Philips HTQ-4 lamp positioned at 15 cm above therotating tube. The tubes obtained had a glass content of 70%. Theresults are given in Table 6. The Examples 40 through 42 serve forcomparison.

                                      TABLE 6                                     __________________________________________________________________________                        state of boundary surfaces                                                              indentation                                                         tackiness hardness                                        Example                                                                            Aromatic ketone(s) (%)                                                                       upper                                                                              under                                                                              upper                                                                             under                                       __________________________________________________________________________    40   1.0% benzil dimethyl ketal                                                                   tacky                                                                              tack-free                                                                          45.9                                                                              50.3                                                                 loose                                                                         fibers                                               41   1.0% benzil    tacky                                                                              tack-free                                                                          49.5                                                                              53.6                                        42   1.00% p-phenyl benzophenone                                                                  tack-free                                                                          tacky                                                                              40.2                                                                              49.0                                                                 loose                                                                         fibers                                               43   0.50% benzil   tack-free                                                                          tack-free                                                                          44.8                                                                              53.0                                             0.50% p-phenyl benzophenone                                              __________________________________________________________________________

EXAMPLES 44 THROUGH 56

An unsaturated polyester resin was obtained by polycondensation of amolar mixture of phthalic anhydride and maleic anhydride with theequivalent amount of propylene glycol up to an acid number of 45 anddissolution in styrene to a content of 34% styrene. In the resin therewere dissolved 1% of aromatic carbonyl compound(s) and 2% oftriisopropanolamine. Subsequently, laminates were formed that were madeup of this resin and four layers of reinforcing glass mat. The laminatethickness was about 4 mm and the glass content 26%. Radiation was donewith a Philips HTQ-4 lamp. The distance between the laminate and thelamp was 20 cm. The results are summarized in Table 7. The Examples 44through 53 serve for comparison. The Examples 54 through 56 serve toillustrate the invention.

These data, too, demonstrate the synergistic effect of the presentinitiator system. They further show the favorable results that may beobtained by the present invention.

                                      TABLE 7                                     __________________________________________________________________________                        state of laminate surface                                                 expo-         indentation                                     aromatic carbonyl                                                                             sure                                                                              tackiness hardness                                        Example                                                                            compound(s) (%)                                                                          (min.)                                                                            upper                                                                              under                                                                              upper                                                                             under                                       __________________________________________________________________________    44   1% benzil dimethyl                                                                       5   tack-free                                                                          tacky                                                                              47.3                                                                              0.0                                              ketal                                                                    45   3% benzil dimethyl                                                                       5   tack-free                                                                          tacky                                                                              46.8                                                                              0.0                                              ketal                                                                    46   1% benzoin butyl                                                                         5   tack-free                                                                          wet  50.7                                                                              0.0                                              ether                                                                    47   3% benzoin butyl                                                                         5   tack-free                                                                          wet  45.2                                                                              0.0                                              ether                                                                    48   1% benzil  3   tack-free                                                                          tacky                                                                              52.5                                                                              0.0                                         49   1% benzil  4   tack-free                                                                          tack-free                                                                          49.5                                                                              7.5                                         50   1% benzil  5   tack-free                                                                          tack-free                                                                          51.2                                                                              21.7                                        51   1% p-phenyl benzo-                                                                       3   tack-free                                                                          wet  0.0 0.0                                              phenone                                                                  52   1% p-phenyl benzo-                                                                       4   tack-free                                                                          wet  0.0 0.0                                              phenone                                                                  53   1% p-phenyl benzo-                                                                       5   tack-free                                                                          wet  0.0 0.0                                              phenone                                                                  54   0.5% benzil                                                                              3   tack-free                                                                          tacky                                                                              46.4                                                                              0.0                                              0.5% p-phenyl benzo-                                                          phenone                                                                  55   0.5% benzil                                                                              4   tack-free                                                                          tack-free                                                                          51.4                                                                              35.9                                             0.5% p-phenyl benzo-                                                          phenone                                                                  56   0.5% benzil                                                                              5   tack-free                                                                          tack-free                                                                          48.2                                                                              53.1                                             0.5% p-phenyl benzo-                                                          phenone                                                                  __________________________________________________________________________

What we claim is:
 1. A photoinitiator mixture comprising:a. benzil,fluorenone, or mixtures thereof; b. a biphenylyl ketone of the formula:##STR5## wherein: R₁ =hydrogen, an alkyl group having 1-8 carbon atoms,an alkoxy group having 1-10 carbon atoms, an alkyl carbonyl group having2-12 carbon atoms or a halogen atom; and R₂ =an alkyl group having 1-12carbon atoms, phenyl, naphthyl, furyl or thiophenyl, which groups may besubstituted or not, andwherein the molar ratio of said biphenylyl ketoneto said benzil, fluorenone, or mixtures thereof is between 0.25 and 9.2. A photoinitiator mixture according to claim 1, characterized in thatthe biphenylyl ketone contained in the mixture is para-phenylbenzophenone.
 3. A photoinitiator mixture according to claim 1 or 2,including a tertiary amine reducing agent.